The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a minute wiring which has sufficiently low contact resistance with a semiconductor substrate and considerably high reliability.
As is well known, films of Al and films of alloys containing Al as a principal component are widely employed as wirings of various kinds of semiconductor device.
However, a variety of problems have recently been experienced as the integration density of semiconductor devices becomes extremely high.
For example, as the integration density becomes higher, the source and drain regions of MOS transistors are reduced not only in area but also in depth. The source and drain regions and the above-described wirings (which may also be called source and drain electrodes) are generally connected to each other as follows. Namely, an insulator film having windows is formed on the surface of a semiconductor substrate, and an impurity is doped into surface regions of the semiconductor substrate through the windows to form source and drain regions. Then, an Al film or an Al-based alloy film is deposited so as to extend over from the surfaces of the source and drain regions to the surface of the insulator film to thereby form wirings. Al and Al-based alloy films are generally deposited by vacuum evaporation effected by means, for example, of electron beam heating . However, when Al and Al-based alloy films are deposited merely by vacuum evaporation, the adhesion of the films to the source and drain regions is weak, so that satisfactory reliability cannot be obtained. Therefore, it isessential to carry out a heat treatment after the deposition of an Al film or an Al-based alloy film in order to increase the adhesion between the deposited film and the source and drain regions. The heat treatment causes an Al-Si alloy layer to be formed at the interface between the wiring film and the semiconductor substrate (the source and drain regions), so that the wiring film and the substrate are connected together strongly and the reliability is thus improved.
However, the Al-Si alloy layer is formed through the phenomenon that Al in the wiring film moves downward and Si in the semiconductor substrate moves upward, and therefore Al undesirably enters the source and drain regions which are formed in the surface region of the semiconductor substrate.
Although the above-described phenomenon gives rise to no particularly serious problem when the integration density of the semiconductor device is relatively low and the source and drain regions have a sufficiently large depth, when the source and drain regions become extremely shallow as described above, Al entering the source and drain regions is undesirably diffused into the p-type semiconductor substrate through the pn junctions defined between the source and drain regions (n.sup.+ -type) and the substrate. Since Al is a p-type impurity, such diffusion of Al causes the pn junctions of the source and drain regions to be destroyed, resulting in the n.sup.30 -type source and drain regions and the p-type semiconductor substrate being undesirably electrically connected to each other.
Such a phenomenon occurs not only in the case where Al or Al-based alloy is employed as a wiring material but also in the case where W, Mo or a silicide of these metals is employed as a wiring material, thus constituting one of serious obstacles to formation of minute wirings. It is a matter of course that such a problem occurs not only in regard of the source and drain regions of MOS transistors but completely the same problem occurs in the case where wirings are connected to any other shallow impurity-doped regions.
In order to solve the above-described problem, a technique in which a TiN film is interposed as a barrier layer between a wiring layer and an impurity-doped region (a semiconductor substrate) has been proposed. Interposition of a TiN film as a barrier layer between them prevents Si from moving from the semiconductor substrate into the metal film and also prevents a metal from moving from the metal film into the semiconductor substrate even if a heat treatment is carried out after the deposition of the metal film. There is therefore no fear of the pn junction of the impurity-doped layer being destroyed, and a semiconductor device of high reliability can be formed.
However, it has been found that employment of a TiN layer as a barrier layer increases the contact resistance in the contact region between the TiN film and the surface of the semiconductor substrate (see Sollar Cells, 9 (1983), pp. 179-183), which means that the proposed technique involves a serious problem in practical use.